The use of effect pigments, also known as pearlescent pigments or nacreous pigments, in order to impart a pearlescent luster, metallic luster and/or multi-color effect approaching iridescent, is well-known. The effect pigments are composed of a plurality of laminar platelets, each of which is coated with one or more reflecting/transmitting layers. Pigments of this type were first based on metal oxides, as described in U.S. Pat. Nos. 3,087,828 and 3,087,829, and a description of their properties can be found in the Pigment Handbook, Volume I, Second Edition, pp. 829-858, John Wiley & Sons, NY 1988. More recently, use of other coating layers to realize optically variable effects have been developed.
The unique appearance of effect pigments is the result of multiple reflections and transmissions of light. The platelet substrate usually has a refractive index which is different from the coating and usually also has a degree of transparency. The coating is in the form of one or more thin films which have been deposited on the surfaces of the platelets.
There are a number of important aspects to effect pigments. One is that they are commonly composed of a plurality of particles which are platelet shaped. If there is a different size or shape, the pearlescent or nacreous appearance is significantly diminished and usually lost to a degree that the material no longer functions as an effect pigment.
One important aspect of the coating on the platelet is that it must be smooth and uniform in order to achieve the optimum pearlescent appearance. The reason is that if an irregular surface is formed, light scattering occurs and the coated platelet will no longer function as an effect pigment.
In addition, the coating should adhere strongly to the platelet or else the coating will become separated during processing, resulting in considerable breakage and loss of luster. Particles which do not become attached to the platelet during preparation of the coatings on the platelets or which are the result of separation cause light scattering and impart opacity to the pigment. When there are too many of such small particles, the pearlescent appearance can be reduced or lost.
The addition of the coatings to a platelet so that the luster, color and color homogeneity are maintained is a very complex process and originally, the only platy substrate which achieved any significant use in commerce was mica. Thus, historically, the largest class of effect pigments based on thin film interference were those based on a mica substrate. With the advent of synthetic substrates, e.g. synthetic mica, aluminum oxide, silica, and glass, it became evident that other substrates could be used since each substrate itself contributes certain effect attributes, due to variations in transparency, refractive index, bulk color, thickness, and surface and edge features. Coated substrate effect pigments thus provide different, albeit similar, visual effects when they are identical except for the identity of the material of the platelet because of these considerations.
Glass flakes are desirable in the industry because they are very resilient and can be optically attractive as well. In one method, glass flakes are made by stretching a molten glass into thin sheets, beads or glass tubes followed by crushing the glass into flakes. The resulting flakes have a size and shape mimicking the mica platelets used in metal oxide-coated mica pearlescent pigments and thus have an average particle size in the range of about 1 to 150 microns and a thickness of about 0.1 to 10 microns.
A commercially viable method of preparing metal oxide-coated glass platelets is described in U.S. Pat. No. 5,753,371, the disclosure of which is hereby incorporated by reference. That patent discloses the coating of C glass in preference to A or E glasses. A glass is a soda-lime glass, commonly used to make windows and contains more sodium than potassium and also contains calcium oxide. C glass, also known as chemical glass, is a form which is resistant to corrosion by acid and moisture. E or electrical glass is, as the name implies, designed for electronic applications and although it is very stable at high temperatures, it can be susceptible to chemical attack.
Metal oxide-coated mica effect pigment and a metal oxide-coated glass effect pigment do provide different visual effects even if they are identical except for the material of the platelet substrate. The reason is that the mica and the glass differ with respect to both their degree of transparency, refractive index, and bulk color. Also, while the surfaces of both are sufficiently smooth for effect pigment use, the glass surface is the smoother of the two substrates and that provides a different optical appearance. Platy aluminum oxide has a surface of similar smoothness to glass. Effect pigments derive their appearance by the reflection and transmission of light and the difference in transparency and refractive index causes the amount of light reflected or transmitted to differ. Nevertheless, both types of effect pigments are highly attractive and commercially valuable.
The preparation of coated glass platelets, while highly desirable, is also expensive. For commercial acceptability, C glass is generally required and this type of glass is costly. In addition, the calcining temperatures employed must be maintained low since the coated glass platelets tend to fuse starting around 650° C. and any significant amount of fusion, generally starting at about 1% by weight of the glass platelets results in the formation of large masses which do not provide the desired pearlescent effect because of their size and irregular shape. Separating the fused platelets from the separate platelets is both time consuming and costly. In addition, the required lower calcining temperature means that the temperature must be maintained for a longer period of time, which also adds to the cost.
Efforts have been made to find a way to reduce the cost of producing the coated glass effect pigment. Theoretically, this could be accomplished by blending coated glass pigment with coated mica pigment. However, this approach has not proven to be effective because the difference in transparency and refractive index between the two platelet materials, in addition to process variations, makes it extremely difficult to match the two blended materials with respect to apparent color. As a practical matter, therefore, it has not been possible to provide a degree of visual homogeneity with a blend which approaches the visual homogeneity of each member of the blend when considered in isolation. This result is not surprising in light of the knowledge in the art. When two or more effect pigments using different substrates are combined together, the attributes of each are present, which results in a unique appearance. One problem with combining effect pigments is that, since the color effects are generated by an additive mechanism instead of a subtractive mechanism, small variations in the color of two effect pigments can result in various degrees of washed out appearance of their blend. This defeats the basic appearance value of the pigment. It can, however, be useful to achieve some other attribute as, for example, to simultaneously achieve an acceptable degree of hiding power and gloss as described in U.S. Pat. No. 6,267,810.
U.S. Pat. No. 5,277,711 describes a mixture of iron oxide-coated aluminum flake and iron oxide-coated mica with or without a prior coating of a colorless, highly refractive metal oxide. The purpose of the iron oxide-coated mica is to reduce the ignition in air and dust explosion hazard otherwise exhibited by the iron oxide-coated aluminum flake. The mixture is made by conjointly coating the aluminum and mica particles with iron oxide in a fluidized bed by gas phase decomposition of iron carbonyls. The appearance of the mixture, homogeneous or otherwise, was not a consideration.
It has now surprisingly been discovered that a visually homogeneous blend of coated effect pigments in which the substrate platelets are of different platy materials can be achieved despite differences in thickness, refractive index and transparency of the platelet materials. It was also surprisingly discovered that with respect to glass platelets premixed with mica, a visually homogeneous product could be made by a process in which the calcining temperature was higher than that employed with coated glass only platelets, thereby reducing the time needed to complete the calcining and further reducing the cost of producing the product.